1. Field of the Invention
The present invention relates to a chopper amplifier circuit capable of reducing a DC offset and noise in an amplifier, and more particularly, to a chopper amplifier circuit capable of reducing an influence of spike noise.
2. Description of the Related Arts
A chopper amplifier circuit has widely been used as a low-noise low-drift DC amplifier.
A conventional chopper amplifier includes, as shown in FIG. 9A, an amplifier 1 and chopper circuits 11 and 12 provided at a preceding stage and a subsequent stage of the amplifier 1, respectively, so as to attain low-noise amplification.
The chopper circuit 11 employs four switching means, which are turned on and off in accordance with pulses φ1 and φ2 shown in FIG. 9B. The pulses φ1 and φ2 are rectangular pulses shifted in phase. The switching means are controlled in a cycle based on the pulses φ1 and φ2 so as to determine which of input signals inputted to input terminals 15 and 16 is inputted to which of a plus (+) input terminal and a minus (−) input terminal of the amplifier 1.
For example, when each of the switches of the chopper circuit 11 is set to be turned on at a pulse of level “H” and to be turned off at a level “L” with no pulse inputted, a line connection status of the circuit changes as follows. Note that the chopper circuit 11 includes switches 11a and 11b which are controlled by the pulse φ1 and switches 11c and 11d which are controlled by the pulse φ2.
Between a time t1 and a time t2, the pulse φ1 is at the level “H” while the pulse φ2 is at the level “L”. Accordingly, the switches 11a and 11b are turned on and the switches 11c and 11d are turned off. In this state, the input terminal 15 is connected to the plus (+) input terminal of the amplifier 1, and the input terminal 16 is connected to the minus (−) input terminal of the amplifier 1.
On the other hand, between the time t2 and a time t3, the pulse φ1 is at the level “L” while the pulse φ2 is at the level “H”. Accordingly, the switches 11a and 11b are turned off and the switches 11c and 11d are turned on. In this state, the input terminal 15 is connected to the minus (−) input terminal of the amplifier 1, and the input terminal 16 is connected to the plus (+) input terminal of the amplifier 1.
Similarly to the chopper circuit 11, the chopper circuit 12 also employs four switching means, which are turned on and off in accordance with the rectangular pulses φ1 and φ2 shifted in phase. The switching means are controlled in a cycle based on the pulses φ1 and φ2 so as to determine which of output signals outputted from a plus (+) output terminal 30 and a minus (−) output terminal 31 of the amplifier 1 is inputted to which of output terminals 17 and 18.
For example, similarly to the chopper circuit 11, when each of the switches of the chopper circuit 12 is set to be turned on at a pulse of level “H” and to be turned off at a level “L” with no pulse inputted, a line connection status of the circuit changes as follows. Note that the chopper circuit 12 includes switches 12a and 12b which are controlled by the pulse φ1 and switches 12c and 12d which are controlled by the pulse φ2.
Between the time t1 and the time t2, the pulse φ1 is at the level “H” while the pulse φ2 is at the level “L”. Accordingly, the switches 12a and 12b are turned on and the switches 12c and 12d are turned off. In this state, the plus (+) output terminal 30 of the amplifier 1 is connected to the output terminal 17, and the minus (−) output terminal 31 of the amplifier 1 is connected to the output terminal 18.
On the other hand, between the time t2 and the time t3, the pulse φ1 is at the level “L” while the pulse φ2 is at the level “H”. Accordingly, the switches 12a and 12b are turned off and the switches 12c and 12d are turned on. In this state, the minus (−) output terminal 31 of the amplifier 1 is connected to the output terminal 17, and the plus (+) output terminal 30 of the amplifier 1 is connected to the output terminal 18.
Next, with reference to FIGS. 10A to 10F, noise and frequency characteristics of input signals at each portion of the conventional chopper amplifier circuit of FIG. 9A will be described. FIGS. 10A to 10F are graphs each showing frequency characteristics at each portion (vertical axis: amplitude, horizontal axis: frequency). Also, FIG. 10G shows the pulses (φ1 and φ2 of FIG. 9B which are inputted to the chopper circuits 11 and 12. In this case, the amplifier 1 has input conversion noise and an offset voltage Vn shown in FIG. 10C. The chopper circuits 11 and 12 each modulate a signal through chopper processing based on the frequency of the pulses φ1 and φ2 (a rectangular wave of frequency fc).
That is, an input signal vin inputted with frequency characteristics of FIG. 10A is subjected to modulation at the chopper circuit 11 based on the pulses φ1 and φ2, so as to be converted into a modulated signal of frequency characteristics shown in FIG. 10B. In this case, the input signal is modulated to have a frequency of an odd-multiple of the frequency of the pulses φ1 and φ2 which control the chopper processing performed in the chopper circuit 11.
Then, in the amplifier 1, the input conversion noise and the offset voltage Vn of FIG. 10C are superimposed on (added to) the modulated signal to be outputted from the amplifier 1 as an amplified signal shown in FIG. 10D. After that, the chopper circuit 12 demodulates the amplified signal into the frequency band of the input signal (low-frequency range including direct current) based on the pulses φ1 and φ2, and outputs the signal as an output signal of frequency characteristics shown in FIG. 1E. At this time, the chopper circuit 12 modulates the input conversion noise and the offset voltage Vn of the amplifier 1 to have a frequency of an odd-multiple of the frequency of the pulses φ1 and φ2 used for the demodulation.
As described above, the output signal outputted from the chopper circuit 12 eventually includes a frequency component of an odd-multiple of the frequency of the pulses φ1 and φ2. In order to remove a high-frequency component included in the output signal, that is, the frequency component of an odd-multiple of the frequency of the pulses φ1 and φ2, a low-pass filter 13 is provided at an output stage, to thereby obtain an output signal having frequency characteristics shown in FIG. 10F (see, for example, P. Allen and D. R. Holberg, CMOS Analog Circuit Design, pp. 490-494, Saunders College Publishing, 1987, hereinafter referred to as Non-Patent Document 1).
In other words, the chopper amplifier circuit described above suppresses an influence of the input conversion noise and the offset voltage Vn of the amplifier 1 to thereby amplify only the frequency component of an input signal.
However, the chopper amplifier circuit described in Non-Patent Document 1 has a drawback in that it is impossible to completely remove spike components included in the output signal through the low-pass filter 13, leading to a harmonic distortion.
In the conventional chopper amplifier circuit, the spike components are generated in the output signal due to the following mechanism.
In the chopper amplifier circuit of FIG. 9A, the input terminal 15 is supplied with an input signal having a sinusoidal wave shown in FIG. 11, while the input terminal 16 is supplied with an input signal having a sinusoidal wave shown in FIG. 12. In each of FIGS. 11 and 12, the vertical axis is a voltage scale and the horizontal axis is a time scale.
The input signal is modulated at the chopper circuit 11, amplified by the amplifier 1, and demodulated at the chopper circuit 12, before being outputted from the output terminal 17 as an output signal. FIG. 13 shows the output signal thus outputted. In FIG. 13, the vertical axis is a voltage scale and the horizontal axis is a time scale.
As is apparent from the waveform of FIG. 13, a large spike components are generated at timings when each of the switches in the chopper amplifier circuits 11 and 12 are switched in accordance with the pulses φ1 and φ2.
The spike components are generated due to a slew rate of the amplifier 1. Specifically, FIG. 14 shows an amplified signal outputted from the plus (+) output terminal 30 of the amplifier 1, and FIG. 15 shows an amplified signal outputted from the minus (−) output terminal 31. In each of FIGS. 14 and 15, the vertical axis is a voltage scale and the horizontal axis is a time scale.
It is evident from FIGS. 14 and 15 that the signal level of the amplified signal significantly fluctuates in voltage when the signal is modulated at the chopper circuit 11.
During the period when the pulse φ1 is at the level “H” and the pulse φ2 is at the level “L”, the chopper circuit 12 samples the amplified signal of FIG. 14 which has been outputted from the plus (+) output terminal 30, and outputs the signal. Alternatively, during the period when the pulse φ1 is at the level “L” and the pulse φ2 is at the level “H”, the chopper circuit 12 samples the amplified signal of FIG. 15 which has been outputted from the minus (−) output terminal 31, and outputs the signal.
In those cases, when the signal is demodulated at the chopper circuit 12, the voltage fluctuation of the amplified signals each outputted from the plus (+) output terminal 30 and the minus (−) output terminal 31, respectively, is synthesized with the demodulated signal because the slew rate of the amplifier 1 is finite. Therefore, the large spike components are generated in the signal.